An absorption heat pump is an environmentally benign replacement for vapor-compression systems using synthetic refrigerants that are typically employed in residential and commercial air-conditioners, chillers and heat pumps. Unlike vapor-compression systems that utilize high-grade electrical or mechanical energy as the input that drives the system, absorption heat pumps run on more readily available and low-grade thermal energy, which can be obtained from combustion of bio-fuels and fossil fuels, from largely untapped waste heat sources, and from solar thermal energy. For example, the heat from the exhaust of a truck or a diesel generator can be used as a heat input.
In a cooling mode operation, the heat input is used to provide cooling and/or dehumidification, while in heating mode operation, the heat input is used to pump ambient heat to higher temperatures. Since in principle the compressor of a vapor-compression system is replaced in an absorption heat pump by the combination of a desorber, an absorber, a liquid solution pump, and a recuperative solution heat exchanger, these absorption heat pumps are more heat and mass exchange intensive, requiring additional transfer surface area and/or heat and mass transfer capacity. Because of the comparatively larger heat and mass transfer rate requirements, absorption heat pumps typically have been associated with large scale commercial and industrial chiller applications.
Achieving compact designs with small footprints while delivering high coefficients of performance (COPs) has been a major challenge thus far. COP can be defined as the ratio of the desired output, cooling or heating, to the input energy. Several advanced absorption cycles such as the double-effect, triple-effect, and Generator-Absorber Heat Exchange cycles, while developed to improve COPs, rely on additional internal recuperation to improve performance, further emphasizing the need for high heat and mass transfer rates per volume. In conventional systems, these cycles have not been widely implemented, especially in small capacity or compact systems, primarily because of the lack of practically feasible and compact heat and mass exchange devices.
In conventional absorption systems that use the two common working fluid pairs, Lithium Bromide-Water and Ammonia-Water, in binary fluid processes, such as absorption and desorption, involve coupled heat and mass transfer in binary fluids. With other less common working fluids, multi-component (more than two species) heat and mass transfer processes are required. In ammonia-water systems, due to the presence of both absorbent (water) and refrigerant (ammonia) in both the liquid and vapor phases throughout the system, including in the condenser, evaporator, rectifier, and recuperative heat exchangers, such binary fluid processes can occur in all components in the system.
For the implementation of absorption systems in compact, high-flux configurations that can take advantage of disperse availability of waste heat or solar thermal energy in smaller capacities than at the industrial scales, the heat and mass exchanger designs usually incorporate several features. These can include low heat and mass transfer resistances for the working fluids, the necessary transfer surface area for the working fluids and the fluids that couple them to external heat sources and sinks in compact volumes, low resistances of the coupling fluids, low working fluid and coupling fluid pressure drop to reduce parasitic power consumption and also losses in driving temperature differences due to decrease in saturation temperatures brought about by pressure drops in components, and even distribution of the working fluid or coupling fluid throughout the component. Typical conventional absorption component concepts fall short in one or more of these criteria important for achieving compact, high-flux designs.